1. Field of the Invention
This invention relates generally to the field of optical fiber data transmission and communication and more particularly concerns an optical interface in the form of an optical interface for interconnecting a 4-channel optoelectronic transmitter-receiver module and an 8-fiber optical fiber transmission ribbon.
2. Background of the Invention
Optoelectronic transmitter-receiver or transceiver modules serve to convert electronic signal to optical signals for transmission via optical fibers and also to convert optical signals received via optical fibers to electronic signals. The signals processed by the optoelectronic transmitter-receiver modules typically consist of digital signals exchanged between two electronic data processing systems or subsystems, for example, in high speed digital data processing systems such as digital telephone switchboards and digital computers. Each system or subsystem is provided with such a module and the two modules are interconnected by an optical fiber transmission cable.
The optoelectronic transmitter-receiver module generally consists of a housing, a number of integrated circuit chips mounted in the housing, a laser diode array driven for emitting pulses of light into the optical fiber cable and a photo-detector diode array for detecting light pulses received from the optical fiber cable. The laser diode array operates as the optical transmitter while the photodetector diode array serves as the optical receiver. The transmitter and the receiver are contained in a common housing or package. Electrical drive signals applied to the laser diodes are converted to light pulses, while light signals received by the photodetector diodes are converted to electrical signals for processing by appropriate electronic circuits in the transceiver module. Each laser diode/photodetector diode supports one channel of communication, and multiple laser diode/photodetector diode arrays support multi-channel communication. Multi-channel optoelectronic transceivers require multiple optical fiber interconnections, a need which is frequently met by use of flat ribbons made up of parallel optical fibers. The optical fiber ribbons are commercially available in different standard widths, most commonly 4-fiber, 8-fiber and 12-fiber ribbons. The ribbons are terminated at opposite ends with corresponding 4-fiber, 8-fiber or 12-fiber optical ribbon connectors, such as industry standard MPO or MTP® optical fiber connectors. The fiber ends of the ribbon lie in a common plane and form a linear array of evenly spaced fiber ends. The termination of optical fiber ribbons represents a significant cost in the manufacture of these systems because the fiber ends in the ribbon must be polished very accurately. For this reason, it is generally desirable to minimize the number of ribbon terminations in a given system.
For example, 4-channel optoelectronic transceivers call for bi-directional 4-channel optical fiber interconnections, that is for an 8-fiberribbon with 4 optical fibers in each direction of the cable. A difficulty arises in the construction of such multi-channel optoelectronic transceivers in that the laser diode transmitter arrays and the photodetector diode receiver arrays consist of separate semiconductor chips which necessarily are mounted at a small distance apart from each other in the transceiver package. The laser and photodetector diodes cannot be manufactured on a single chip in order to maintain even spacing between all diodes and thus match the even spacing between adjacent fibers across the width of the optical fiber ribbon. If the laser diode and the photodiode are mounted adjacently, then the light from the laser enters the adjacent photodiode as a result of reflection or the like, and crosstalk may arise. In a four channel transceiver the array of four transmitter diodes and the array of four receiver diodes are separated by a gap and for this reason the individual diodes cannot be aligned with the eight more closely spaced fibers ends of an 8-fiber optical ribbon termination. It is possible to provide two separate 4-fiber ribbons each with its own 4-fiber connector aligned with one of the 4-diode arrays, one ribbon for connecting the 4-diode transmitter array of one transceiver to the 4-diode receiver array of the opposite transceiver; and another ribbon for connecting the 4-diode receiver array of the one transceiver to the 4-diode transmitter array of the opposite transceiver. However, this solution requires four ribbon terminations on the two separate ribbons. It is more desirable both from a viewpoint of economics as well as simplicity of construction and maintenance to provide a single 8-fiber ribbon for interconnecting two 4-channel transceivers. The temperature of heat resistance of the fiber ribbon is about 100° C. If a high temperature treatment such as the solder reflow for the module is applied, the ribbon may be damaged.